Cavitation is a phenomena which occurs in fluids when the pressure in the fluid drops below the vapor pressure and occurs in fire engine pumps when the hose water demand exceeds the water available. Cavitation manifests itself by the presence of gas in the fluid. When cavitation is extensive a large void ratio is created in the fluid which may result in instability, vibration and noise. The presence of cavitation in an engine driven pump is known to create pitting of the internal parts of the pump. Even more importantly, mild cavitation reduces the flow rate discharge and pressure of the pump and, if these quantities are control parameters, the speed of the engine may be increased to return them to the designed level. The increase in engine speed, (and pump speed) however, further aggrevates the condition by further reducing fluid pressure and increasing cavitation. Thus, the pump can "lose" water and the flow rate drops dramatically, sometimes to zero.
Fire pumper trucks are now in general use and are designed to increase the pressure of water available at a hydrant and deliver it through a plurality of hoses to firemen (nozzlemen) who are located by a fire. Generally, the site of the fire is quite remote from the pumper, and thus hoses of up to several hundred feed are utilized.
As the fire is being sprayed, the nozzlemen relay commands back to the pumper for increases and decreases in flow rate. As the total flow rate to the nozzlemen is increased in response to commands, the speed of engine driven pump is increased. Now, if the demand for water exceeds the available supply, it can result in pump cavitation and a drop in flow rate. If cavitation occurs, water will be supplied in reduced quantity to the nozzlemen and may even cease. It will be readily appreciated that the loss of water may trap the nozzlemen in the fire with no means of escape.